[LegibleCrimson2]

Ah, yeah, you're right. I was spacing the fact that C as well as C++ can have multiple calling convention. I blame early morning brain.

As far as the wrong calling convention goes, I'm basing it on the fact that an extern "C++" function can be passed as a callback where an extern "C" is demanded. Even if they're the same calling convention, that should fail, but it doesn't. Looks like it doesn't fail at runtime, which is a small comfort, but given the different permissiveness of different compilers, it still makes me very nervous to pass a C++ function as a C callback and just hope that it works, given that it isn't guaranteed in the standard.

[spacechild1]

> Even if they're the same calling convention, that should fail, but it doesn't.

It's an interesting question. According to the standard, functions with different language linkage are indeed considered different types. As a consequence, <cstdlib> should declare two overloads for qsort() that only differ in the type of the sort function. However, modern compilers don't seem to care:

"The only modern compiler that differentiates function types with "C" and "C++" language linkages is Oracle Studio, others do not permit overloads that are only different in language linkage, including the overload sets required by the C++ standard"

https://en.cppreference.com/w/cpp/language/language_linkage

In practice, extern "C" does two things (as you correctly pointed out):

1. disable name mangling - This only affects the symbol name and is not relevant for callback functions

2. enforce the (default) C calling convention - On all (modern) platforms I know, C and C++ have the same default calling convention for free functions.

This means that from the view of a C++ compiler, pointers to `foo()` and `extern "C" foo()` have the exact same type.

Anyway, no need to be nervous. Even if the compiler treated these as different types, you would get a compiler error because C++ disallows implicit casts between different pointer types.

[LegibleCrimson2]

As long as I can't silently get wrong behavior or runtime crashes, I'm happy enough. Is it guaranteed that an incorrect calling convention will always cause a compiler error? I wasn't aware the calling convention was considered part of the pointer type.

Anyway, thanks for engaging with me so earnestly. I guess I had some assumptions about calling conventions that needed to be straightened out, which is important, as I'm doing work in this territory right now.

[spacechild1]

> Is it guaranteed that an incorrect calling convention will always cause a compiler error?

A standard-conforming C++ compiler must not allow implicit pointer casts, so yes!

> I wasn't aware the calling convention was considered part of the pointer type.

Some well-designed C APIs define a macro for the calling convention that they add to all API functions and function pointer declarations. The user can then use the same macro when supplying their callbacks, which guarantees that the calling conventions match. (On modern platforms, the macro would be typically empty.)

Here's an example: https://github.com/Celemony/ARA_API/blob/1f68fba7a374b14df19.... As you can see, it is part of the function pointer type: https://github.com/Celemony/ARA_API/blob/1f68fba7a374b14df19...

Another famous example is, of course, the WINAPI macro in the Win32 API.

That's also what I tend to do with my own C APIs.

> I guess I had some assumptions about calling conventions that needed to be straightened out

I also learned a few things in this discussion, so thanks for that!